As one of a back frame of a vehicle seat, there is a gate-shaped back frame which serves as a framework of a seat back and has a substantially inverted U-shape in a front view and which is formed by connecting upper end portions of a pair of side frames extending in an upper and lower direction in a bridging manner with an upper frame. Such type of a back frame disclosed in JP-A-2014-156212 is used for a vehicle seat with a built-in three-point seatbelt. In the structure of the back frame disclosed in JP-A-2014-156212, an upper end portion of a high-strength side frame on one side to which a load from a webbing of a seatbelt device is directly applied and an upper end portion of a normal-strength side frame on the other side are connected by an upper frame, and a belt guide is attached to the upper end portion of the high-strength side frame. Further, a lower end side of the high-strength side frame and a lower end side of the normal-strength side frame are connected in a bridging manner by a lower pipe. The belt guide is a member for guiding a webbing released from a webbing take-up device disposed inside a seat back so as to extend from a back side toward a front side of the back frame.
In the above structure, the lower pipe connects the lower end side of the high-strength side frame and the lower end side of the normal-strength side frame and functions to suppress the gate-shaped back frame having a substantially inverted U shape in a front view not to fall in a left and right direction. However, the lower pipe does not have a function of enhancing the bending rigidity of the high-strength side frame and/or the normal-strength side frame in a front and rear direction. Thus, it has been difficult to reduce the weight of the back frame by reducing an external line shape of the cross section of the high-strength side frame and/or the normal-strength side frame or reducing thickness of component members of the high-strength side frame and/or the normal-strength side frame. Therefore, a structure capable of enhancing the bending rigidity of the high-strength side frame and/or the normal-strength side frame in the front and rear direction with a simple structure has been required.